Labelled 29-hydroxy fucosterol will be fed to Achlya to determine how well it is incorporated into the oogoniols. 29-Oxo fucosterol will also be fed in order to find out more about its role in the biosynthetic scheme. The sequence in which hydroxyl groups are introduced in the tetracyclic nucleus will be studied by feeding experiments with 11 alpha, 29-dihydroxy fucosterol and 15 Beta,29-dihydroxy fucosterol. We hope also to learn at what stage introduction of the 7-ketone and esterification of the C-3 hydroxyl occur. Our objective is to define more clearly the biosynthetic pathway to the oogoniols.